The biggest difference between an bank and a credit union is the bank is in it to make money. They are a corporation trying to make cash by using your money. So any interest you draw for money you have held with said bank is them paying you for letting them use your money.
Credit Unions are more of a community based outfit, and they are more or less non-profit. Any money made by the Credit Union is share back out with the members of it based on a percentage of how much money you have invested in the credit union. More money the credit union made that year the higher % you get.
My experience is that credit unions pay better interest, have lower credit card charges, and as a member I have some input into decisions. I have found banks to be less flexible also.